Semiconductor laser driving circuit, light emitting device, and disk drive

ABSTRACT

A semiconductor laser driving circuit has a circuit protection function at low temperature and includes a voltage current converter that converts an input voltage Vin, which is determined according to a desired light brightness of the semiconductor laser to be driven, into a current. A current limiter limits an output current of the voltage current converter to a specified current value or less. An output amplifier amplifies the output current of the voltage current converter and supplies the amplified current as a drive current to the semiconductor laser. A temperature detection circuit detects a low temperature state and, in the low temperature state, decreases the specified current value of the current limiter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser driving techniqueand, more particularly, to a driving technique in a low temperaturerange of a nitride semiconductor laser.

2. Description of Related Art

In recent years, a wideband gap semiconductor laser diode made of amaterial such as gallium nitride (GaN), which is the material for a bluelaser is being used more and more. Such a nitride semiconductor lasercharacterized by a short wavelength is widely applied to a pickup devicefor writing/reading data to/from a next-generation DVD (DigitalVersatile Disc), a display, a printer, medical equipment, and the like.

Generally, the brightness of a semiconductor laser is determinedaccording to the amount of current flowing in the device. Therefore, tomake a semiconductor laser emit light with a desired brightness, it isnecessary to perform constant current driving by supplying a currentfrom a driving circuit and according to the desired brightness (refer toJapanese Patent Laid-Open No. H5-259544 and Japanese Utility ModelLaid-Open No. S63-29968).

Although the properties of an arsenic (As)-based or phosphorus (P)-basedsemiconductor laser emitting red light or near infrared light improve asthe temperature decreases, the nitride semiconductor laser has a problemin that its properties deteriorate as the temperature decreases. Thisproblem is caused by a large band gap and a deep impurity level used fordoping of the nitride semiconductor. Specifically, since the impuritylevel is deep, the activation rate of carries (holes in the case of anitride semiconductor) is low even at room temperature. When thetemperature further decreases, the carrier concentration becomes lower.Generally, the electric conductivity of a semiconductor is determined bythe product of the carrier mobility and carrier concentration. When thecarrier concentration decreases at the time of low temperature, deviceresistance increases. Further, in a junction region in a diode, thecarriers on the side where the decrease in the carrier concentration islarger (a P-type region in the nitride semiconductor laser) tend todeplete, so that carriers of the opposite polarity (electrons in thenitride semiconductor laser) move over the junction region to the sidewhere the carrier concentration is low (a P-type region in the nitridesemiconductor laser). The carriers injected to the opposite polarityregion where resistance became higher cause energy obtained from anelectric field generated due to the high resistance release in the formof a point defect, and it causes problems such as failure in the deviceand deterioration in reliability such as device life span.

Generally, the brightness of a semiconductor laser is controlled bycurrent injection control using the driving circuit as described above.Consequently, when a current which is the same as that at roomtemperature is supplied to a semiconductor laser in which a resistancevalue increases at a low temperature, a voltage applied to the devicebecomes high at the low temperature. In some cases, this negativeinfluence is exerted on the device characteristics, the life span, andthe like.

It is expected that driving of a semiconductor laser, particularly inconsumer products, is started in a low-temperature environment.Considering that next-generation DVDs on which nitride semiconductorlasers are mounted and the like are being widely sold and used in thefuture, it is necessary to enhance circuit protection of a semiconductorlaser.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a semiconductor laser driving circuithaving the function of circuit protection at low temperature.

According to a preferred embodiment of the present invention, a drivingcircuit for driving a semiconductor laser includes a voltage currentconverter arranged to convert an input voltage, which is determinedaccording to a desired light brightness of the semiconductor laser to bedriven, into a current, a current limiter arranged to limit an outputcurrent of the voltage current converter to a specified current value orless, an output amplifier arranged to amplify the output current of thevoltage current converter and to supply the amplified current as a drivecurrent to the semiconductor laser, and a temperature detection circuitarranged to detect a low temperature state and to decrease the specifiedcurrent value of the current limiter in the low temperature state.

Since the resistance value of a wideband gap semiconductor laser such asa nitride semiconductor laser increases as the temperature decreases,when the same current is passed at the room temperature and at the lowtemperature, the voltage applied to the device at the low temperaturebecomes high. In this mode, consequently, by limiting the currentflowing in the semiconductor laser to a predetermined current at lowtemperature, application of a high voltage to the semiconductor lasercan be prevented, and suitable circuit protection can be performed. The“low temperature state” denotes a state of a temperature at which thecharacteristics of the semiconductor laser deteriorate and varies amongdevices. Although a clear temperature range is not specified, the lowtemperature may be, for example, 10° C. or less, or 0° C. or less.

The temperature detection circuit may determine the low temperaturestate on the basis of an operation voltage of the semiconductor laser tobe driven. Further, the temperature detection circuit preferablyincludes a comparator arranged to compare an operation voltage of thesemiconductor laser to be driven with a predetermined threshold voltageand, when the operation voltage exceeds the threshold voltage,determines a low temperature state.

In the case of driving the semiconductor laser with a constant current,the operation voltage, that is, the anode-to-cathode voltage, increasesas the temperature decreases. Therefore, by monitoring the operationvoltage of the semiconductor laser, it can be determined that thesemiconductor laser is in the low temperature state.

The temperature detection circuit may decrease the specified currentvalue of the current limiter as the operation voltage of thesemiconductor laser to be driven increases.

By decreasing the upper limit of the current flowing in thesemiconductor laser as the operation voltage of the semiconductor laserincreases, that is, as the temperature of the device decreases, moresuitable circuit protection can be performed.

The temperature detection circuit may include an inverting amplifierarranged to amplify the difference between the operation voltage of thesemiconductor laser to be driven and a predetermined reference voltageand, on the basis of an output voltage of the inverting amplifier, maydecrease the specified current value of the current limiter.

In this case, the output voltage of the inverting amplifier rises as thetemperature decreases. Consequently, as the temperature decreases, theupper limit value of the drive current can be lowered.

The temperature detection circuit may include a transistor having oneend at which a potential is fixed and the other end to which a constantcurrent load is connected, and a bias circuit for applying a constantvoltage to the gate of the transistor. The temperature detection circuitmay determine a low temperature state in accordance with an on/off stateof the transistor.

A gate-source threshold voltage Vt of a MOSFET (Metal OxideSemiconductor Field Effect Transistor) rises as the temperaturedecreases. By applying a constant voltage according to the thresholdtemperature for determining the low temperature state to the gate, theMOSFET turns off at a low temperature and turns on at a hightemperature, so that a low temperature state can be determined.

The temperature detection circuit may include a temperature sensorarranged to monitor the temperature of the semiconductor laser to bedriven. By using the temperature sensor, the temperature of thesemiconductor laser can be directly measured.

The voltage current converter may preferably include a first resistorhaving a first end at which a potential is fixed, a first transistorhaving a first end connected to a second end of the first resistor, asecond resistor having a first end at which a potential is fixed, asecond transistor having a first end connected to a second end of thesecond resistor, and a first operational amplifier having anon-inversion input terminal to which a voltage according to the inputvoltage is applied and which is connected to a connection point of thefirst resistor and the first transistor, and having an output terminalconnected to control terminals of the first and second transistors, andmay output current flowing to the second transistor. The current limitermay preferably include a third resistor having a first end at which apotential is fixed, a third transistor having a first end is connectedto a second end of the third resistor, and a second operationalamplifier having a non-inversion input terminal to which a voltageaccording to the specified current value is applied and which isconnected to a connection point of the third resistor and the thirdtransistor, and having an output terminal connected to control terminalsof the first, second, and third transistors.

In this case, when a voltage according to the input voltage, which isinput to the non-inversion input terminal of the first operationalamplifier is lower than a voltage according to the specified currentvalue, which is input to the non-inversion input terminal of the secondoperational amplifier, a current proportional to the voltage accordingto the input voltage is output. When the voltage according to the inputvoltage exceeds the voltage according to the specified current value,the output current is limited to the specified current value.

The driving circuit may be integrated on a single semiconductorsubstrate. The “integrating” includes the case where all of componentsof the circuit are formed on a semiconductor substrate and the casewhere main components of the circuit are integrated. A portion ofresistors, capacitors, and the like may be provided on the outside ofthe semiconductor substrate for adjusting a circuit constant. Byintegrating the driving circuit as one LSI, the circuit area can bereduced, and the characteristics of the circuit device can be maintaineduniformly.

Another preferred embodiment of the present invention relates to a lightemitting device. The light emitting device preferably includes asemiconductor laser, and the above-described driving circuit for drivingthe semiconductor laser. The semiconductor laser may be a nitridesemiconductor laser made of GaN or other suitable material.

Further another preferred embodiment of the present invention relates toa disk drive. The disk drive includes the above-described light emittingdevice for irradiating a disk medium with light output from thesemiconductor laser.

An arbitrary combination of the components and replacement of thecomponents and expressions of various preferred embodiments of thepresent invention described above among the methods, devices, systems,and the like are also effective as embodiments of the present invention.

With the driving circuit according to various preferred embodiments ofthe present invention, the device can be protected excellently at a lowtemperature.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the configuration of a lightemitting device according to a first preferred embodiment of the presentinvention.

FIG. 2 is a circuit diagram showing an example of a detailedconfiguration of a driving circuit in FIG. 1.

FIG. 3 is a circuit diagram showing an example of the configuration of avoltage current converter, a current limiter, and an output amplifier.

FIG. 4 is a diagram showing input/output characteristics of the drivingcircuit in FIG. 1.

FIG. 5 is a circuit diagram showing the configuration of a drivingcircuit according to a second preferred embodiment of the presentinvention.

FIG. 6 is a diagram showing input/output characteristics of the drivingcircuit of FIG. 5.

FIG. 7 is a block diagram showing the configuration of a disk driveusing the driving circuit according to a preferred embodiment of thepresent invention.

FIG. 8 is a circuit diagram showing another configuration example of atemperature detection circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First PreferredEmbodiment

FIG. 1 is a circuit diagram showing the configuration of a lightemitting device 200 according to a first preferred embodiment of thepresent invention. The light emitting device 200 preferably includes asemiconductor laser LD1 and a driving circuit 100 for driving thesemiconductor LD1. In the present preferred embodiment, the drivingcircuit 100 is preferably integrally formed as a functional IC on asingle semiconductor substrate. The driving circuit 100 generates anoutput current Iout according to an input voltage Vin, which isdetermined according to a desired light brightness of the semiconductorlaser LD1 to be driven, and supplies the output current Iout to thesemiconductor laser LD1. The driving circuit 100 has, as input/outputterminals, an input terminal 102, an output terminal 104, and a groundterminal 106. The input voltage Vin is supplied from the outside to theinput terminal 102. The output terminal 104 is connected to the anode ofthe semiconductor laser LD1 to be driven, and the ground terminal 106 isgrounded together with the cathode of the semiconductor laser LD1. Inthe present preferred embodiment, current flowing in the semiconductorlaser LD1 is called the output current Iout (or drive current), and ananode-to-cathode voltage in the semiconductor laser LD1 is called anoperation voltage Vop. In the present preferred embodiment, thesemiconductor laser LD1 is preferably a nitride semiconductor laser madeof GaN or other suitable material.

A voltage-current converter 10 converts the input voltage Vin, which isdetermined according to a desired light brightness of the semiconductorlaser LD1 to be driven, into a current Idrv1. A current limiter 20limits the output current Idrv1 of the voltage-current converter 10 to aspecified current value Ilim or less. An output amplifier 30 amplifiesan output current Idrv2 of the voltage-current converter 10 limited bythe current limiter 20 and supplies the resultant current as a drivecurrent Iout to the semiconductor laser LD1.

A temperature detection circuit 40 monitors a temperature Temp of thesemiconductor laser LD1 and detects a low temperature state of thesemiconductor laser LD1. The temperature detection circuit 40 outputs acontrol signal S1 to the current limiter 20. When the temperaturedetection circuit 40 detects the low temperature state, the temperaturedetection circuit 40 causes the specified current value Ilim of thecurrent limiter 20 to decrease.

FIG. 2 is a circuit diagram showing an example of a detailedconfiguration of the driving circuit 100 in FIG. 1 according to thepresent preferred embodiment. The voltage-current converter 10preferably includes a resistor R10 and a conductance amplifier 12. Oneend of the resistor R10 is grounded and the other end of the resistorR10 is connected to the input terminal 102. A non-inversion inputterminal of the conductance amplifier 12 is connected to the inputterminal 102. The input voltage Vin is applied to the non-inversioninput terminal of the conductance amplifier 12. The conductanceamplifier 12 converts the input voltage Vin to the current Idrv1 andoutputs the current Idrv1.

In the present preferred embodiment, the temperature detection circuit40 preferably includes a comparator 42 and a reference voltage source44, and determines a low temperature state on the basis of the operationvoltage Vop of the semiconductor laser LD1 to be driven. Thenon-inversion input terminal of the comparator 42 is connected to theoutput terminal 104, and the operation voltage Vop is supplied to thecomparator 42. A threshold voltage Vth1 generated by the referencevoltage source 44 is supplied to the non-inversion input terminal of thecomparator 42. The comparator 42 compares the operation voltage Vop ofthe semiconductor laser LD1 with the threshold voltage Vth1 and outputsthe control signal S1 which becomes the high level when Vop>Vth1 andbecomes the low level when Vop<Vth1. When the operation voltage Vopexceeds the threshold voltage Vth1, the temperature detection circuit 40determines the low temperature state.

In the case of driving the semiconductor laser LD1 with a constantcurrent, the lower the temperature is, the higher the operation voltageVop is. The driving circuit 100 according to the present preferredembodiment determines that the semiconductor laser is in the lowtemperature state by monitoring the operation voltage Vop of thesemiconductor laser LD1.

The current limiter 20 sets the specified current value Ilim to a firstcurrent Ilim1 when the control signal S1 becomes the low level, and setsthe specified current Ilim to a second current Ilim2 lower than thefirst current Ilim1 when the control signal S1 becomes the high level.

FIG. 3 is a circuit diagram showing an example of the configuration ofthe voltage current converter 10, the current limiter 20, and the outputamplifier 30.

The voltage current converter 10 preferably includes a first resistorR1, a second resistor R2, a first transistor M1, a second transistor M2,a first operational amplifier OP1, and a resistor R10.

One end of the first resistor R1 is grounded and the potential of thefirst resistor R1 is fixed. The first transistor M1 is preferably anN-channel MOSFET and its source is connected to the other end of thefirst resistor R1. Similarly, one end of the second resistor R2 isgrounded, and the source of the second transistor M2 as an N-channelMOSFET is connected to the other end of the second resistor R2. Theinput voltage Vin is supplied to the non-inversion input terminal of thefirst operational amplifier OP1, and the inversion input terminal of thefirst operational amplifier OP1 is connected to a connection point ofthe first resistor R1 and the first transistor M1. The output terminalof the first operational amplifier OP1 is connected to the gate as acontrol terminal of each of the first and second transistors M1 and M2.The voltage current converter 10 outputs the current flowing in thesecond transistor M2 as the drive current Idrv2.

In the voltage current converter 10, feedback control is performed sothat the potential at the non-inversion input terminal of the firstoperational amplifier OP1 and that at the inversion input terminalbecome close to each other, so that the potential at the connectionpoint between the first resistor R1 and the first transistor M1 becomesequal to the input voltage Vin. As a result, current given byIdrv1=Vin/R1 flows in the first resistor R1 and the first transistor M1.

In the present preferred embodiment, the first and second resistors R1and R2 are paired, and the first and second transistors M1 and M2 arealso paired. As a result, the gate potential of the first and secondtransistors M1 and M2 becomes equal to the output voltage of the firstoperational amplifier OP1, and the current Idrv2 according to the drivecurrent Idrv1 flows in the second transistor M2.

The current limiter 20 preferably includes a third resistor R3, a thirdtransistor M3, a second operational amplifier OP2, and a voltage source22. One end of the third resistor R3 is grounded, and the potential ofthe third resistor is fixed. The source of the third transistor M3 isconnected to the other end of the third resistor R3. The voltage source22 generates the specified voltage Vlim according to the specifiedcurrent Ilim. The voltage source 22 is a variable voltage source. Thecontrol signal S1 is input to the voltage source 22. When the controlsignal S1 is at the low level, the specified voltage Vlim is set to thefirst voltage Vlim1. When the control signal S1 is at the high level,that is, in the low temperature state, the voltage source 22 sets thespecified voltage Vlim to the second voltage Vlim2 lower than the firstvoltage Vlim1. The specified voltage Vlim output from the voltage source22 is input to the non-inversion input terminal of the secondoperational amplifier OP2, and the inversion input terminal is connectedto the connection point of the third resistor R3 and the thirdtransistor M3. The output terminal of the second operational amplifierOP2 is connected to the gate as a control terminal of each of the first,second, and third transistors M1, M2, and M3.

When the input voltage Vin becomes higher than the specified voltageVlim, the current limiter 20 becomes active. When the current limiter 20becomes active, feedback control is performed so that the potential atthe non-inversion input terminal of the second operational amplifier OP2and that at the inversion input terminal become equal to each other. Asa result, the potential at the connection point of the third resistor R3and the third transistor M3 becomes equal to the specified voltage Vlim,and the current determined by Ilim=Vlim/R3 flows in the third transistorM3 and the third resistor R3.

In the present preferred embodiment, the third and second transistors M3and M2 are paired, and the third and second resistors R3 and R2 are alsopaired. As a result, the gate potential of the second and thirdtransistors M2 and M3 becomes equal to the output voltage of the secondoperational amplifier OP2 when the current limiter 20 is active, and thecurrent Idrv2 according to the current Ilim flows in the secondtransistor M2 when Vin>Vlim.

By action of the current limiter 20, the current Idrv2 can be limited tothe specified current Ilim or less according to the specified voltageVlim. By changing the specified voltage Vlim, the upper limit value ofthe output current Iout can be adjusted.

The output amplifier 30 preferably includes transistors M10 and M11 asP-channel MOSFETs. The transistor M10 is provided on the path of thedrive current Idrv2. The gates and sources of the transistors M10 andM11 are commonly connected, thereby defining a current mirror circuit.The transistors M10 and M11 amplify the drive current Idrv2 inaccordance with the mirror ratio, and output the output current Ioutfrom the output terminal 104. When the mirror ratio is 1:n, the equationof Iout=Idrv2×n is satisfied.

The operation of the driving circuit 100 with such a configuration willbe described. FIG. 4 shows the input/output characteristics of thedriving circuit 100 according to the present preferred embodiment. Theaxis of abscissa of FIG. 4 shows the input voltage Vin and the axis ofordinate indicates the output current Iout. When the control signal S1output from the temperature detection circuit 40 is at the low level,that is, when the temperature is not low, the specified voltage Vlim ofthe current limiter 20 is set to Vlim1. The output current Idrv2 of thecurrent limiter 20 increases in proportion to the input voltage Vin inthe range of Vin<Vlim1. When the input voltage Vin becomes higher thanthe specified voltage Vlim1 (Vin>Vlim1), the current limiter 20 becomesactive, and the current Idrv2 is limited to Ilim1 or less. As a result,the output current Iout of the driving circuit 100 is limited to n×Ilim1or less.

When the temperature detection circuit 40 detects the low temperaturestate, the control signal S1 becomes the high level, and the specifiedvoltage Vlim generated by the voltage source 22 of the current limiter20 becomes the second voltage Vlim2. As a result, the upper limit valueof the output current Iout is limited to n×Ilim2 or less.

Since the resistance value of the wideband gap semiconductor laser suchas a nitride semiconductor laser increases as the temperature decreases,when the same current is passed at the room temperature and at the lowtemperature, the voltage applied to the device at the low temperaturebecomes high. In the driving circuit 100 according to the presentpreferred embodiment, by changing the upper limit value of the currentflowing in the semiconductor laser LD1 at the low temperature and at theroom temperature, application of a high voltage to the semiconductorlaser can be prevented, and suitable circuit protection can beperformed.

Since the resistance value of the semiconductor laser LD1 increases asthe temperature decreases, the operation voltage Vop at the time ofperforming constant current driving by passing constant current to thesemiconductor laser LD1 increases as the temperature decreases.Consequently, the temperature detection circuit 40 of the drivingcircuit 100 according to the present preferred embodiment can preferablydetermine the low temperature state by monitoring the operation voltageVop.

Second Preferred Embodiment

FIG. 5 is a circuit diagram showing the configuration of the drivingcircuit 100 according to a second preferred embodiment of the presentinvention. The driving circuit 100 according to the second preferredembodiment is different from that of the first preferred embodiment withrespect to the configuration of the temperature detection circuit 40.The point different from the first preferred embodiment will be mainlydescribed hereinbelow.

In the driving circuit 100 according to the second preferred embodiment,the temperature detection circuit 40 makes the specified current Ilim ofthe current limiter 20 decrease as the operation voltage Vop of thesemiconductor laser LD1 to be driven increases. The temperaturedetection circuit 40 outputs a control signal S2 for controlling thespecified current value Ilim to the current limiter 20.

The temperature detection circuit 40 includes the reference voltagesource 44, resistors R41 and R42, and an operational amplifier OP4. Areference voltage Vref generated by the reference voltage source 44 issupplied to the non-inversion input terminal of the operationalamplifier OP4. A resistor R41 is provided between the inversion inputterminal of the operational amplifier OP4 and the output terminal 104,and a resistor R42 is provided between the output terminal and theinversion input terminal of the operational amplifier OP4. Theoperational amplifier OP4 and the resistors R41 and R42 construct aninverting amplifier.

The inverting amplifier including the operational amplifier OP4 and theresistors R41 and R42 amplifies the difference between the operationvoltage Vop of the semiconductor laser LD1 to be driven and thepredetermined reference voltage Vref. An output voltage Vs2 of theinverting amplifier is output as the control signal S2 to the currentlimiter 20.

In the present preferred embodiment, the current limiter 20 sets aspecified voltage Vlim in accordance with the output voltage Vs2 of theinverting amplifier. For example, the output voltage Vs2 of theoperational amplifier OP4 may be input to the non-inversion inputterminal of the second operational amplifier OP2 of the current limiter20 in FIG. 3 directly. Alternately, the output voltage Vs2 is multipliedby a constant, and the resultant voltage may be input to thenon-inversion input terminal.

FIG. 6 shows input/output characteristics of the driving circuit 100according to the second preferred embodiment. In the present preferredembodiment, the specified voltage Vlim that sets the specified currentIlim of the current limiter 20 continuously changes in accordance withthe operation voltage Vop. Specifically, when the temperature decreasesat the time of driving the semiconductor laser LD1 with constantcurrent, the operation voltage Vop increases, and the output voltage Vs2of the temperature detection circuit 40 decreases. As a result, as thetemperature decreases, the specified voltage Vlim and the specifiedcurrent Ilim decrease.

In the driving circuit 100 according to the second preferred embodiment,as the operation voltage Vop of the semiconductor laser LD1 increases,that is, as the temperature of the device decreases, the upper limitvalue of the current flowing in the semiconductor laser LD1 can bedecreased. Thus, preferable circuit protection can be achieved.

Finally, an application example in which the light emitting device 200according to the first and second preferred embodiments can bepreferably used will be described. FIG. 7 is a block diagram showing theconfiguration of a disk drive 300 using the driving circuit 100according to the present preferred embodiment. The disk drive 300preferably includes a spindle motor 330 for rotating a disk medium 400,a motor controller 320 for driving the spindle motor 330, the lightemitting device 200 according to a preferred embodiment of the presentinvention, a DSP 310, and a not-shown pickup. The DSP 310 is a block forcontrolling the disk drive 300 in a centralized manner, controls theemitting state of the light emitting device 200 and also controlsrotation of the spindle motor 330. The DSP 310 outputs the input voltageVin to the light emitting device 200. The light emitting device 200causes the semiconductor laser LD1 emit light in accordance with theinput voltage Vin, and irradiates the disk medium 400 with light outputfrom the semiconductor laser LD1 to execute reading/writing data.

It is understood by a person skilled in the art that the preferredembodiments are illustrative, the components and processes of thepreferred embodiments can be variously modified, and the modificationsare within the range of the present invention.

In the first preferred embodiment, the temperature detection circuit 40may have the configuration shown in FIG. 8. FIG. 8 is a circuit diagramshowing another configuration of the temperature detection circuit 40.The temperature detection circuit 40 preferably includes a transistorM40, a constant current source 48, a band gap reference circuit 46, andresistors R44 and R45. The transistor M40 is preferably an N-channelMOSFET, its source is grounded, and the potential is fixed. The constantcurrent source 48 as a constant current load is connected to the drainof the transistor M40. The band gap reference circuit 46 generates aconstant voltage Vref. The resistors R44 and R45 divide the constantvoltage Vref and apply the resultant voltage to the gate of thetransistor M40. The temperature detection circuit 40 determines the lowtemperature state in accordance with the on/off state of the transistorM40.

A gate-source threshold voltage Vt of the MOSFET increases as thetemperature decreases. Consequently, by preliminarily applying theconstant voltage according to the threshold temperature for determiningthe low temperature state to the gate, the MOSFET turns off at lowtemperature and on at high temperature. Therefore, the low temperaturestate can be determined.

The temperature detection circuit 40 may include a temperature sensorfor monitoring the temperature of the semiconductor laser LD1 to bedriven. By directly monitoring the temperature of the semiconductorlaser LD1 by the temperature sensor and controlling the value of thespecified current Ilim in the current limiter 20, circuit protection canbe realized.

The driving circuit 100 may be integrated or part of the driving circuit100 may be constructed by a discrete component or a chip component. Thepart of integration or the degree of integration may be determinedaccording to the specifications, cost, occupation area, and the like ofthe driving circuit 100.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A driving circuit for driving a semiconductor laser, comprising: a voltage current converter arranged to convert an input voltage, which is determined according to a desired light brightness of the semiconductor laser, into a current; a current limiter arranged to limit an output current of the voltage current converter to a specified current value or less; an output amplifier arranged to amplify the output current of the voltage current converter and to supply the amplified current as a drive current to the semiconductor laser; and a temperature detection circuit arranged to detect a low temperature state and to decrease the specified current value of the current limiter in the low temperature state.
 2. The driving circuit according to claim 1, wherein the temperature detection circuit determines the low temperature state on the basis of an operation voltage of the semiconductor laser.
 3. The driving circuit according to claim 2, wherein the temperature detection circuit includes a comparator arranged to compare an operation voltage of the semiconductor laser with a predetermined threshold voltage and, when the operation voltage exceeds the threshold voltage, determines a low temperature state.
 4. The driving circuit according to claim 2, wherein the temperature detection circuit decreases the specified current value of the current limiter as the operation voltage of the semiconductor laser increases.
 5. The driving circuit according to claim 4, wherein the temperature detection circuit includes an inverting amplifier arranged to amplify the difference between the operation voltage of the semiconductor laser and a predetermined reference voltage and, on the basis of an output voltage of the inverting amplifier, decreases the specified current value of the current limiter.
 6. The driving circuit according to claim 1, wherein the temperature detection circuit includes a transistor having a first end at which a potential is fixed and a second end to which a constant current load is connected, and a bias circuit for applying a constant voltage to the gate of the transistor, and the temperature detection circuit determines a low temperature state in accordance with an on/off state of the transistor.
 7. The driving circuit according to claim 1, wherein the temperature detection circuit includes a temperature sensor arranged to monitor a temperature of the semiconductor laser to be driven.
 8. The driving circuit according to claim 1, wherein the voltage current converter includes: a first resistor having a first end at which a potential is fixed and a second end; a first transistor having one end that is connected to the second end of the first resistor; a second resistor having a first end at which a potential is fixed and a second end; a second transistor having one end that is connected to the second end of the second resistor; and a first operational amplifier having a non-inversion input terminal to which a voltage according to the input voltage is applied and which is connected to a connection point of the first resistor and the first transistor, and having an output terminal connected to control terminals of the first and second transistors, and outputs current flowing to the second transistor; wherein the current limiter includes: a third resistor having a first end at which a potential is fixed and a second end; a third transistor having one end connected to the second end of the third resistor; and a second operational amplifier having a non-inversion input terminal to which a voltage according to the specified current value is applied and which is connected to a connection point of the third resistor and the third transistor, and having an output terminal connected to control terminals of the first, second, and third transistors.
 9. The driving circuit according to any claim 1, wherein the driving circuit is integrated on a single semiconductor substrate.
 10. A light emitting device comprising: a semiconductor laser; and the driving circuit according to claim 1 arranged to drive the semiconductor laser.
 11. The light emitting device according to claim 10, wherein the semiconductor laser is a nitride semiconductor laser.
 12. A disk drive comprising the light emitting device according to claim 10, for irradiating a disk medium with light output from the semiconductor laser. 